Light: polarization
Principle
In electrodynamics, polarization is the property of electromagnetic waves,
such as light, that describes the direction of their transverse electric field.
More generally, the polarization of a transverse wave describes the direction
of oscillation in the plane perpendicular to the direction of travel.
Longitudinal waves, such as sound waves, (see Sound and Acoustics) do not
exhibit polarization, because for these waves the direction of oscillation is
along the direction of propagation.
The simplest manifestation of polarization to visualize is that of a plane
wave, which is a good approximation to most light waves (a plane wave is a wave
with infinitely long and wide wave fronts). All electromagnetic waves
propagating in free space or in a uniform material of infinite extent have
electric and magnetic fields perpendicular to the direction of propagation.
Conventionally, when considering polarization, the electric field vector is
described and the magnetic field is ignored since it is perpendicular to the
electric field and proportional to it. The electric field vector may be
arbitrarily divided into two perpendicular components labeled x and y with z indicating the direction of travel, see Fig. 1. The x
vector is at the left (red) and the y-vector at the right (green). For a simple
harmonic wave (sinus), the two components have the same frequency, but the
amplitude and phase may differ. In the example of Fig. 1, the projection of the
movement of the electric vector (blue in Fig. 1) on the horizontal x-y plane creates
the purple line (the most simple Lissajous figure).
Fig. 1 Linear polarized light.
In Fig. 1, the two orthogonal (perpendicular) components are in phase.
Since the ratio of the strengths of the two components is constant, the
direction of the electric vector (the vector sum of these two components) is
constant. And since the tip of the vector traces out a single line on the plane
perpendicular on the direction of propagation, this special case is called linear polarization. The direction of
this line depends on the relative amplitudes of the two components.
Application
Light reflected by shiny transparent
materials is partly or fully polarized, except when the light is normal
(perpendicular) to the surface. A polarizing filter, such as a pair of
polarizing sunglasses, can be used to observe this by rotating the filter while
looking through. At certain angles, the reflected light will be reduced or
eliminated. Polarizing filters remove light polarized at 90o to the
filter's polarization axis. If two polarizers are placed atop one another at 90
o angles to one another, no light passes through.
Linear polarization is used in polarization microscopy.
Polarization by scattering is observed as light passes through the
atmosphere. The scattered light produces the brightness and blue color in clear
skies. This partial polarization of scattered light can be used to darken the
sky in photographs, increasing the contrast (Fig. 2). This effect is easiest to
observe at sunset, on the horizon at a 90 o angle from the setting
sun.
Fig. 2 Left unpolarized, right
partly polarized. The effects of a polarizer on the sky in a color photograph.
The right picture has the polarizer, the left does not.
Brightness of images of the sky and clouds reflected from horizontal
surfaces are drastic reduced, which is the main reason polarizing filters are
often used in sunglasses.
Rainbow-like patterns, visible through polarizing sunglasses, are caused by
color-dependent birefringent (double
refraction, see Light: refraction) effects, for example in toughened glass (e.g. car windows) or items made
from transparent plastics. The role played by polarization in the operation of
liquid crystal displays (LCDs) is also frequently apparent to the wearer of
polarizing sunglasses, which may reduce the contrast or even make the display
unreadable.
Biology
The naked human eye is weakly sensitive to polarization. Polarized light
creates a very faint pattern near the center of the visual field, called
Haidinger's brush. With sun light this pattern, a yellow horizontal figure 8
shaped spot with a bleu spot above and below the figure 8, is very difficult to
see, but with practice one can learn to detect polarized light with the naked
eye.
Many animals are apparently capable of perceiving the polarization of
light, which is generally used for navigational purposes, since the linear
polarization of sky light is always perpendicular to the direction of the sun.
This ability is very common among the insects, including bees, which use this
information to orient their communicative dances. Polarization sensitivity has
also been observed in species of octopus, squid, cuttlefish, and mantis shrimp.
This ability is based on the polarizing effect of the anisotropic
microstructure of the photoreceptor cells of these animals.
Fig. 3 Circular polarization.
More info
In Fig. 3 the two orthogonal components have exactly the same amplitude and
are exactly (plus or minus) 90o out of phase. In this special case
the electric vector traces out a circle in the plane of projection, so this
special case is called circular polarization. The direction of rotation depends
on the sign of the phase difference.
All other cases, that is where the two components are not in phase and
either do not have the same amplitude and/or are not 90o out of
phase are called elliptical polarization because the electric vector traces out
an ellipse in the horizontal plane.
In nature, electromagnetic radiation is often produced by a large number of
individual radiators, producing waves independently of each other. In general
there is no single frequency but rather a spectrum of different frequencies
present, and even if filtered to an arbitrarily narrow frequency range, there
phases and planes of polarization are different. This type of light is
described as incoherent. However, this does not mean that polarization is
only a feature of coherent radiation. Incoherent radiation may show statistical
correlation between the components of the electric field, which can be
interpreted as partial polarization.
In general it is possible to describe a beam of light as the sum of a
completely incoherent part (no correlations) and a completely polarized part.
One may then describe the light in terms of the degree of polarization, and the parameters of the polarization
ellipse.
A more complete description, fundamental and mathematically can be found in
Wikipedia.